Fixed the crasher in it.

This reverts commit 21fa170f.

Is crashing.  Oops.

Hopefully fixes bots.

Fixes https://github.com/harfbuzz/harfbuzz/issues/345

appveyor.yml: try updating msys2 to fix failing mingw-w64-x86_64 build

maybe it'll get a bit faster

so it won't attempt reinstalling packages which are already installed

https://github.com/harfbuzz/harfbuzz/pull/1093#issuecomment-405201903
https://ci.appveyor.com/project/harfbuzz/harfbuzz/build/job/ky7lao1ii1bi7ew4#L71

It uses constexpr...

Also, include hb-dsalgs.hh from end of hb-pricate.hh

Oops.

Catches compile-errors in them better.

gen-def.py: pass headers as arguments so that msys2 can convert posix paths

let python's parse command-line args as usual

When one is not using the msys2 python, the header files that are passed in as environment
variable cannot be found.

https://ci.appveyor.com/project/fonttools/ttfautohint-py/build/1.0.65/job/rkremny4jjid9nl2#L803

This is because msys2 shell and make use POSIX paths (e.g. /c/Users/clupo/...)
whereas non-msys2 python.exe uses native Windows paths (e.g. C:\Users\clupo\...).

Msys2 will automatically convert command line arguments (but not environment variables) from
POSIX to Windows paths when calling a native win32 executable, so we pass the header paths
as arguments instead of environment variables.

This way the gen-def.py script can support both mingw python running in an MSYS2 shell, and
native win32 python.

Fixes #481